1. Technical Field
The present invention relates generally to analog-to-digital (A/D) converters and, more particularly, to a folding-type A/D converter.
2. Background Art
For a wide variety of systems that process information digitally, the need to convert analog signals to digital signals is essential. Consequently, analog-to-digital (A/D) converters of many types have been developed to convert the analog signal to a digital representation of this signal. Generally, a typical A/D converter will generate a transfer function that divides the analog signal into a plurality of amplitude levels and produces digital codes of n-bit length representing the respective amplitude levels of the analog signal.
A folding-type A/D converter is one type of well-known converter that is used in many systems. The folding-type A/D converter produces a digital code that has the property of "unit distance", which property means that only one bit changes at a time as the amplitude of the analog signal changes from one level to the next, as will be described more fully below. This type of converter typically uses several signal processors that process an analog input signal by generating a transfer function which divides the input signal range into a number of linear segments depending on the number, n, of bits in the code. For example, for a code of n-bit length, there are n linear segments. Each linear segment corresponds to a bit in the code from the least significant bit to the most significant bit, and has an amplitude range corresponding to the weight or significance of each such bit. For example, and as will also be further described below, the linear segment corresponding to the least significant bit, e.g., bit 0, will have one amplitude range, the linear segment corresponding to the next significant bit, or bit 1, will have another amplitude range, etc.
As will be further described also, the converter is a "folding-type" in that the linear segments for the respective n-1 most significant bits are folded or repeated over the range of 2.sup.n bit transition levels L. For example, in a 4-bit code, there are 16 bit transition levels L over which the linear segments for the respective n-1 most significant bits are repeated. The linear segment corresponding to the most significant bit, however, does not repeat, i.e., there is only one, non-repetitive linear segment corresponding to the most significant bit.
U.S. Patent No. 4,058,806 discloses a folding-type A/D converter which directly converts an analog voltage input signal to a digital code. In this patent, the folded linear segments of the transfer function for a given bit are generated in the voltage domain, i.e., as voltages, by using diodes that are connected in AND-OR configurations. These generated folded segments are then directly compared with a threshold voltage by a comparator, whereby the output of the comparator is a logic 1 or logic 0 for the given bit. One problem with this prior A/D converter is that the diodes used to generate the folded linear segments must match exactly; otherwise, there will be a degradation in the linearity of the folded linear segments. In addition, since this converter operates in the voltage domain, multiple amplifiers or, alternatively, multiple buffers, are used in order to mitigate undesirable loading that is caused by the diode network, thereby increasing the number of components, cost and complexity of the converter.
Another folding-type A/D converter is disclosed in a publication entitled "Fast ADC", by Arbel and Kurz, IEEE Transactions on Nuclear Science, Vol. NS-22, February, 1975. In this converter, a given linear segment of the transfer function is generated and repeated for a bit of given significance by utilizing current differencing amplifiers, each of which responds to an input analog voltage input signal. A given differential pair of amplifiers is connected such that as the input voltage signal passes through a range of .DELTA.V volts, an output current is produced corresponding to the linear segment. By connecting a number of these differential pairs of amplifiers as described in this publication, segment folding or repeating is obtained in increments of .DELTA.V volts. The resulting output currents of these differential pairs of amplifiers, representing the repeating segments of the transfer function for a given bit, are then converted to a voltage and fed to an external "flash" A/D converter where the folded signal is then digitized.
While the folding-type A/D converter of Arbel and Kurz operates in the current domain for generating the transfer function and, therefore, does not have the problems associated with the above-mentioned U.S. Pat. No. 4,058,806 which operates in the voltage domain, the former does have several disadvantages. First, the analog voltage input signal is actively processed, i.e., amplified, by the differential pairs of amplifiers. In addition to requiring a large number of differential amplifiers, this active processing of the input signal greatly increases the possibilty of error. Secondly, the digital code is not produced directly from the linear segments of the transfer function. Rather, a given segment is digitized by the external "flash" A/D converter, thereby requiring additional circuitry for the overall A/D conversion process. Still furthermore, the overall voltage range of the converter of Arbel and Kurz is determined largely by the response range of the differential pairs of amplifiers. Consequently, the initial setup of this converter requires fine-tuning for obtaining precise .DELTA.V ranges, which is difficult to accomplish. Still furthermore, even if the initial setup is accomplished successfully, these differential amplifiers are prone to temperature drift, thereby affecting the precision of .DELTA.V.